It's arbitrary, there is no fundamental reason to scale scores one way or the other.
I think having the scores shown relative to the maximum achievable for the map is more convenient than the system other game modes use. For example, a score above 950k in osu!mania is always a good indicator of a certain level of proficiency in the map, whereas in osu! a score of 5 million in osu! standard can be anything between very poor and perfect, depending on the map, (also, there is the issue that score is a very poor indicator of performance overall in osu! standard)